Science 3360

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Science 3360

• Lecture 21 Tropical Phenomenon
• An examination of the El Nino/Southern
  Oscillation system and an overview of hurricanes.

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El Nino
The name El Nino was originally given to a warm
current that appeared off the cost of Peru and
Ecuador. The current only flowed for a few
weeks, and, because it usually occurred near
Christmastime, local fisherman named it after the
Christ child. El Nino now refers to an abnormal
warming of surface ocean waters in the eastern
tropical Pacific. El Nino is closely coupled
with the Southern Oscillation, a seesaw pattern
of reversing air pressure between the Eastern and
Western tropical Pacific. Because the ocean
warmings and pressure reversals are nearly
simultaneous, this phenomenon is most correctly
called the El Nino/Southern Oscillation or
ENSO. To understand ENSO, we must first look at
sea surface temperatures (SSTs)
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Average Wintertime SSTs
Ocs.orst.edu
Warmest waters occur in the western equatorial
Pacific Ocean. What would you expect to occur
there?
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Normal Conditions
Regions of intense convection occur off the coast
of Australia and Indonesia. The rising air flows
northward (remember the Hadley Cell) as well as
east-west. The westerly (from the west) wind
blows across the Pacific and subsides off the
coast of South America. The circulation is
completed by the easterly flow at the surface.
Notice that when pressure is high in the W.
Pacific, it tends to be low in the
Eastern/Central Pacific. This is the Southern
Oscillation. Where would you expect more
precipitation?
Typical Wintertime Flow
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Normal Conditions
Thermocline at a higher level in the E. Pacific
means that the surface layer is thin. This
allows nutrient-rich cold water to upwell. This
leads to large ocean productivity and fish
populations off the W. coast of South America.
This cold water extends over the E. Pacific and
is called the cold tongue.
The warm water piles up in the W. Pacific. This
forms the so-called warm pool. This piling of
water raises the sea-level in the W. Pacific,
leading to a slope downward from W to E.
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Normal Conditions
The Tropical Pacific ocean-atmosphere system is a
classic example of the chicken and the egg. Is
the ocean forcing the atmosphere? The atmosphere
forcing the ocean? Neither, it is a single,
integrated system.
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La Nina
La Nina represents a strengthening of the normal
conditions in the tropical Pacific . It is
sometimes called the negative phase of ENSO.
The SSTs in the E. Pacific are colder than normal
and may extend further westward. The convection
in the Western Pacific is also intensified.
La Nina events are sometimes called cold
events. Why? Because the cold pool is colder
and larger.
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El Nino
There is considerable debate about what occurs to
trigger an ENSO event. We will avoid the
controversy and just ask the question what
would happen if the easterly surface winds weaken
(or in some cases, reverse direction)?
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El Nino

• Without the easterly surface winds, theres
  nothing to hold back the warm pool. The water
  flows back toward the central Pacific, taking
  about 60 days.
• There are two major events when this occurs
• The warm pool shifts to the central Pacific.
• This shifts the area of maximum convection to the
  East.
• The upwelling off the west coast of South America
  shut down as the thermocline shifts.
• This leads to a drastic die-off of marine life
  and the people and birds that feed off them.

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El Nino
So now pressure increases over the Western
Pacific and decreases in the Central Pacific.
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El Nino
We can quantify this pressure seesaw (that
represents the Southern Oscillation) by looking
at the pressure over Tahiti (central Pacific) and
Darwin, Australia (western Pacific).
The Southern Oscillation Index (SOI) is
calculated from the sea-level pressure
differences. Negative values of the SOI
represent warm (El Nino) events. Note the
strengths of the 1982-83 and 1997-98 events.
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El Nino
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El Nino Why Do We Care?
What are the climatic effects of El Nino? The
most dramatic global manifestations of ENSO are
in rainfall patterns. In non-Enso years,
convective activity occurs over Australia and
Indonesia as well as the Amazon basin and
equatorial Africa. During ENSO years, the
convective center shifts to the central Pacific.
Australia and Indonesia get much drier. There
are also non-local effects that result in
droughts in central America and northern Brazil.
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El Nino
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El Nino in America
These are the average, typical effects. In
reality, each ENSO event is unique in spatial
manifestation and duration.
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Hurricanes
Tropical Cyclones are a generic term for a low
pressure system that forms in the tropics. The
cyclone is accompanied by thunderstorms and in
the Northern Hemisphere, a counterclockwise
circulation of winds. Why do the winds spiral
counterclockwise? Tropical Cyclones are
classified as follows Tropical Depression an
organized system of clouds and thunderstorms with
a defined surface circulation and maximum
sustained winds of 38mph or less Tropical Storm
a storm with the same characteristics as a
tropical depression but with winds between
39-73mph. Hurricanes an intense tropical
weather system of strong thunderstorms with a
well-defined surface circulation and maximum
sustained winds of 74mph or higher. On average,
11 tropical storms develop over the Atlantic
Ocean, Caribbean Sea, and Gulf of Mexico. On
average, six of these will intensify and become
hurricanes. In other parts of the world,
hurricanes are known by different names.
Typhoons occur in the Western North Pacific and
Tropical cyclones occur in the Indian Ocean and
Western South Pacific. These are all the same
phenomenon.
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Hurricanes
Hurricanes are categorized according to the
strength of their winds using the Saffir-Simpson
Hurricane Scale. This scale was developed by
civil engineer Herbert Saffir and meteorologist
Bob Simpson who at the time was director of the
National Hurricane Center. The SSHS was
developed to mirror the Richter scale. The U.S.
National Hurricane Center classifies hurricanes
of category 3 and above as major hurricanes.
The SSHS has been criticized as being too
simplistic in that it does not take into account
the size of the storm nor the quantity of
precipitation that it may produces.
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Hurricanes
The most recognizable feature found within a
hurricane is the eye, usually 20-50km in
diameter. The eye is the location of the lowest
surface pressure. Skies are often clear above
the eye and winds are relatively light. The eye
is calm because the strong surface winds are
deflected away from the center due to the
Coriolis effect. The area just outside the eye
is the eye wall, the location of the most
damaging winds and the most intense rainfall.
Eye walls are called as such because oftentimes
the eye is surrounded by a vertical wall of
clouds.
Hurricane Isabel 2003
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Hurricanes
Radiating outward from the eye wall, one can see
banded structures within the clouds. These
clouds are called spiral rain bands. Normally
if one were to travel from the outer edge of the
hurricane to the center, one would progress from
light rain to dry conditions back to slightly
more intense rain again over and over until
reaching the eye.
From ww2010.atmos.uiuc.edu
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Hurricanes How They Form
Atlantic / Caribbean Hurricanes often begin as
thunderstorms off the coast of Africa. Why off
the coast of Africa? Scientists arent totally
sure but it may have to do with dust coming off
the Sahara Desert (CCN). These storms travel
westward in the Intertropical Convergence Zone.
Not all storms grow to become tropical
depressions/storms/hurricanes. What is needed to
further their development?
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Hurricanes How They Form
If the storm passes over ocean water that has a
warm (gt80F), thick layer on the top, it may
intensify. What role does the warm water
provide? Why is a thick layer of warm water
needed? The storm takes energy from the warm
water and heats up the surface air. The warm air
is convected upward where it cools (condensation
may occur) and comes back down. This cycle takes
place many times, creating a vertical system over
the ocean. If conditions remain favorable, the
storm can continue to develop and strengthen.
What sort of conditions would cause the storm
to weaken or dissipate?
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Tropical Depression
A tropical depression is designated when the
thunderstorms begin to organize and surface
pressures are consistently low. When viewed from
a satellite, tropical depressions appear to have
little organization. However, the slightest hint
of rotation can usually be perceived when looking
at a series of images. Winds near the center are
constantly between 23 and 39mph.
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Tropical Storm
Once a tropical depression has intensified to the
point where its maximum sustained winds are
between 39 and 73mph, it becomes a tropical storm
and is assigned a name. During this period,
the storm itself becomes more organized and
begins to become more circular in shape, starting
to resemble a hurricane.
Tropical Storm Fay, August 17, 2008
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Hurricanes
As the surface pressure continues to drop, a
tropical storm becomes a hurricane when the
maximum sustained winds reach 74mph. A
pronounced rotation occurs around the central the
core, the eye. The eye is unique to hurricanes.
Katrina, 2005
Rita, September 2005
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Hurricanes
What determines a hurricanes path? Hurricanes
are pushed along by the background flow much as a
leaf is carried by a stream. Near the equator
the easterly trade winds push hurricanes to the
west with a slight nudge toward the north. If
the storm travels far enough north, the
mid-latitude westerlies will push the storm to
the east.
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Hurricanes
Why do hurricanes die out? If you take away the
energy source and add more friction, the storm
will slow and dissipate. If the storm moves
over land, what happens? The source of water
vapor dries up the source of energy (heat)
disappears and the increased drag slows the
storm down. In general, air flowing over land
experiences more drag than over water (the
scientific term is surface roughness). The
hurricane may regain strength if it moves back
over a region favorable for development. This is
often seen over the Gulf of Mexico as storms make
landfall over Cuba or Florida, lose strength,
then regain strength as they move back over warm
water. What about hurricanes that never make
landfall? Eventually the storm moves over cold
water, effectively shutting off the energy
source. Strong upper level winds (wind shear)
can also rip apart the upper level structure of
the hurricane, disrupting the convective
cells. The average lifespan of a hurricane is 9
days.
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Hurricane Damage
As we all know, hurricanes can cause widespread
catastrophic damage. The move obvious cause of
damage is high winds which can uproot trees,
knock over buildings, and hurl debris at high
speeds. The wind speeds for a hurricane are
not constant for all areas as landfall is made.
Land
Weaker Winds Stronger Winds
Direction of Hurricane travel
Ocean
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Hurricane Damage
Another major source of hurricane-related damage
is the storm surge. Storm surge is the rising of
sea level due to high winds and low pressure.
Strong winds blowing toward the shore push
water towards shore on the right side of the
hurricanes direction of motion. This
contributes to most of the coastal flooding.
Also, the central pressure of a hurricane is so
low that the relative lack of atmospheric
pressure above the eye and eye wall causes a
bulge in the ocean surface level. Ocean waves
also contribute to the overall storm surge as the
waves which may only be a few meters out at sea
grow to several meters at the shore due to the
shallower coastline. Storm surge usually varies
between 1 and 5 meters. For an area with flat
topography, the storm surge can intrude up to
several miles.
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Hurricane Damage
Hurricanes also produces tremendous amounts of
rain which can cause both flash and long-term
flooding.
Hurricanes can also spawn tornados. These are
produced in the spiral rain bands as or after the
hurricane makes landfall.